The process of reverse osmosis (RO) desalination of raw water includes basically the following:                providing a semi-permeable membrane (RO membrane) which allows passing of the solvent (water) molecules and rejects dissolved salts, and disposing the RO membrane in a suitable vessel;        feeding the raw water to one side of the RO membrane (“feed” side) under gauge pressure higher than the osmotic pressure of the raw water;        collecting permeated desalinated water from the other side of the membrane (“permeate” side); and        removing concentrated saline water (“brine”) from the feed side (also called “feed-brine” side).        
Raw water used as a source for desalination by reverse osmosis, besides dissolved solids (salts), includes suspended particles, organic and mineral, collectively called suspended solids. Standard desalination technology includes a filtration system called “pretreatment” system which is designated to remove the suspended solids. Pretreatment systems remove a significant part of the suspended solids, but are not able to remove them absolutely. Some of the suspended solids pass the pretreatment system, reach the RO membrane and stick to its feed surface as “foulants”.
The foulants accumulated on the membrane surface provide favorable conditions for growth of bacteria. This is referred to as bio-fouling, and is the most brutal form of fouling.
Some dissolved salts may also precipitate on the membrane surface, forming scale. The RO desalination process is designed according to the solubility limits of sparingly soluble salts. Sometimes super-saturation conditions can occur. Desalination close to the saturation limits may cause formation of micro-crystals, which serve as nucleation centers. These micro-crystal particles typically don't cause problems, but under certain conditions can grow and create severe scaling of the membrane.
As seen from the above, not only suspended solids may foul the membrane surface. Consequently, in this description, raw water after pretreatment will be called raw saline solution, and will be considered to comprise solvent (water), dissolved salts and a potential foulant.
A fouled membrane has reduced separability of the dissolved salts and increased pressure losses and, therefore, has to be cleaned.
Common methods for cleaning the foulants from the feed membrane side involve stopping the RO desalination process and pumping cleaning chemical solutions along the feed-brine side to wash the fouled membrane surface. Such methods are costly, not very effective, and create environmental problems.
More effective methods for membrane cleaning are based on using direct osmosis (DO) for creating permeate backwash flow through the RO membrane. Such flow can lift the fouling from the membrane surface so that it can be removed with the brine stream.
Publications EP0768112 and JP2000-079328 disclose implementations of the direct osmosis method of membrane cleaning. They suggest that when the RO membrane is cleaned, the RO process of permeate production is stopped, but the raw saline solution is still supplied to the feed side of the membrane, essentially without pressure (or at least under pressure lower than the osmotic pressure of the solution). Permeate is supplied to the permeate side of the membrane also without pressure.
Patent application WO2004/062774 suggests increasing the gauge pressure at the permeate side of the membrane to a level slightly below the feed pressure of raw saline solution. This is done by back pumping the permeate for a few seconds. As the osmotic pressure of the raw saline solution is much higher then the osmotic pressure of the permeate, the net balance of pressures provides power for the direct osmosis process and membrane cleaning. This method avoids stopping of feed pumps and/or decreasing the feed side pressure so that the RO process can be easily resumed. However, this method requires that high-pressure permeate pump and expensive stainless-steel permeate ports, permeate collection piping system, high pressure valves, etc. are employed instead of low-cost plastic equipment that is usually used at the permeate side. Hardware replacement may increase significantly the cost of desalinated water.